1. Field of the Invention
The present invention relates to a method of making an optical glass article, especially useful as an optical transmission line. More particularly, it relates to a method of making a solid or hollow cylindrical optical glass article having a refractive index profile in a radial direction which is formed by controlled addition of fluorine.
2. Description of the Prior Arts
Hitherto, several methods have been proposed to form a refractive index profile in an optical glass article by controlled addition of fluorine.
According to the first method, the refractive index profile is formed during vapor phase axial deposition of glass soot in the VAD method, which is schematically shown in FIG. 1. In this method, a burner 61 is used for supplying a glass-forming raw material (e.g., silicon tetrachloride and the like) for synthesizing glass soot which forms a core part 64 of the glass soot preform. A burner 62 is used for supplying a glass-forming raw material and a fluorine-containing compound for synthesizing glass soot which forms a peripheral part 65 of the glass soot preform. Numeral 63 stands for a glass soot preform. According to this method, fluorine diffuses and is inevitably homogeneously added to the glass soot preform, so that a refractive index profile is always uniform.
The second method is schematically shown in FIG. 2. In this method, a glass soot preform 73 is made in such manner that a core part 74 contains a compound for lowering a vitrification temperature of glass (e.g., GeO.sub.2, P.sub.2 O.sub.5 and the like) in a larger concentration than a peripheral part 75. When the glass soot preform 73 is inserted in a furnace 77 kept at high temperature, since the core part 74 shrinks faster than the peripheral part 75, it is possible to add more fluorine to the peripheral part 75 so as to form a refractive index profile by selecting a concentration of the fluorine-containing compound in a heating atmosphere and timing of the addition of the fluorine-containing compound. However, the second method has a disadvantage that the glass article contains materials other than silica and fluorine although it can realize the refractive index profile.
The third method is described in Preprint 2-183 for 1984 National Meeting of Denshi-Tsushin Gakkai, Hikari Denpa Bumon (Electronic Communication Society, Light and Electric Wave Section) and so on. In this method, a glass soot preform formed so that a core part and a peripheral part have different bulk densities. When fluorine is added to such glass soot preform a furnace kept at high temperature, more fluorine is added to the peripheral part than to the core part since it diffuses more easily into the peripheral part than into the core part due to difference of the bulk density. Thereby, refractive index difference is made between the core and peripheral parts. To achieve 0.3 % of refractive index difference between the core and peripheral parts according to the third method, the peripheral part should have a bulk density of about 0.2 g/cm.sup.3 and the core part should have a bulk density larger than 1.5 g/cm.sup.3. However, it is extremely difficult to remove residual water from the glass soot preform having a bulk density so large as 1.5 g/cm.sup.3.
The fourth method is proposed in Japanese Patent Kokai Publication (unexamined) No. 61-132532. In this method, the refractive index profile is realized by providing glass soot particles with a particle size variation in a radial direction of the glass soot preform so as to vary the vitrification temperature of the glass soot preform in the radial direction, whereby an amount of added fluorine is controlled. Although the fourth method has several advantages such that the transparent glass preform which contains less residual water and no other material than SiO.sub.2 and fluorine can be obtained, and the additional fluorine content can be changed in the radial direction, it requires special technique for achieving the particle size variation such as utilization of plasma flame, carbon monoxide flame and the sol-gel method.
The above described four methods are developed to realize the refractive index profile in the optical glass article. On the contrary, it is sometimes desired to make the refractive index profile uniform. However, it is difficult to realize a completely uniform profile of the refractive index in the optical glass article according to the above conventional methods, since the variation of the bulk density of the glass soot preform in the radial direction influences the amount of fluorine to be added.